A method is known in which a layer with high quality is obtained employing a mixed gas of rare gas and a layer formation gas according to an atmospheric pressure plasma discharge treatment method, however, this method results in cost increase, since helium or argon which is employed as the discharge gas, is expensive. When a cheap gas other than rare gas, for example, an oxygen gas, a nitrogen gas or carbon dioxide in air, is used as a discharge gas in this method, stable discharge does not occur under a conventional high frequency electric field and a uniform layer is difficult to be formed, since the strength (hereinafter referred to also as electric field intensity) of electric field starting discharge is high.
A method is disclosed in Japanese Patent O.P.I. Publication No. 10-154598 in which even a gas having a high discharge starting electric field intensity such as a nitrogen gas can induce discharge due to application of a pulse electric field. However, in this method, plasma density is low, a formed layer is poor in quality, and the layer formation speed is low, resulting in low productivity.
A method is disclosed in Japanese Patent O.P.I. Publication No. 11-16696 in which an oxygen gas or a mixed gas of an oxygen gas and inert gas is activated or ionized between preliminary electrodes by application of low frequency voltage, the activated or ionized gas is supplied together with an oxygen gas or a mixed gas of an oxygen gas and inert gas, which is not activated nor ionized, to a space between main electrodes which are provided in parallel with the preliminary electrodes, high frequency voltage is applied across the main electrodes to generate plasma at atmospheric pressure and generate active species, and a substrate is surface treated in which the surface of the substrate is etched with the active species or an organic substance on the surface of the substrate is subjected to ashing with the active species.
A method is disclosed in Japanese Patent O.P.I. Publication No. 2002-110397 in which an argon gas is used as a discharge gas, and an electric field in which a pulsed high frequency electric field and a pulsed direct current electric field are superposed is applied to an electrode on one side, whereby stable discharge can be produced to form a layer on a substrate.
A method for loading an electronic component is disclosed in Japanese Patent O.P.I. Publication No. 11-191576 in which a nitrogen gas is used, and an electric field in which a high frequency electric field and a low frequency electric field are superposed is used, whereby plasma is generated, and a substrate is washed with the generated plasma.
However, it has been found that when the method disclosed in Japanese Patent O.P.I. Publication No. 11-16696 is used for forming a layer, that is, an oxygen gas or a mixed gas of an oxygen gas and inert gas is activated or ionized between preliminary electrodes by application of low frequency electric field, the activated or ionized gas is mixed with another layer formation gas, and the resulting mixture gas is supplied to a space between main electrodes and high frequency voltage is applied across the main electrodes, particles are produced and no layer is not formed. Further, it has been found that the method comprising mixing an oxygen gas in plasma state and a layer formation gas has danger of explosion, which is not suitable for a layer formation method.
A layer formation method is disclosed in Japanese Patent O.P.I. Publication No. 2002-110397 in which after plasma is generated by application of direct pulsed electric field and the generated plasma is stabilized by application of a high frequency electric field replacing the direct pulsed electric field, a substrate is introduced into the stabilized plasma, wherein the direct pulsed electric field and the high frequency electric field are not superposed. It has been found that the electric field application described above cannot provide a layer with high performance.
Japanese Patent O.P.I. Publication No. 11-191576 only discloses a method for washing an electronic component, in which the high frequency electric field and the low frequency electric field are superposed. It has been found that a simple superposing of the high frequency electric field and the low frequency electric field is difficult to form a layer with high performance.